Zn1-xHgxSe films with x ranging from 0.2 to 1 were prepared by electrochemical deposition from aqueous baths at 25-85 degrees C. Both bath composition and deposition potential were varied over a wide range. Large variations of the mercury content of the films can be realized by appropriate choice of the deposition potential while for a given deposition potential the mercury content increases steeply with increasing mercury concentration in the bath and tends towards a limiting value. The deposition potential is therefore the chief parameter permitting a predetermined film composition to be established. For a series of films deposited at potentials between -0.5 and -1.0 V vs. SCE, the lattice parameter a of the cubic ternary compound obtained from the X-ray diffractograms (XRD) gives a stoichiometric factor x matching closely the mercury content (at.%) found by electron probe microanalysis (EPMA). This Hg content (at.%) can therefore be directly related to the stoichiometric factor x showing that essentially all mercury is incorporated in the single phase, Zn1-xHgxSe. The crystallinity (crystal size) improves with increasing x. This is accompanied by more pronounced orientation of the crystallites in the (111) direction. HgSe is almost completely oriented. The photoelectrochemical response of the films in Na-polysulphide solution shows distinct n-type behaviour.